The unidirectional airflow patterns in the lungs of birds have long been considered a unique and specialized trait associated with the oxygen demands of flying, their endothermic metabolism1 and unusual pulmonary architecture2,3. However, the discovery of similar flow patterns in the lungs of crocodilians indicates that this character is probably ancestral for all archosaurs—the group that includes extant birds and crocodilians as well as their extinct relatives, such as pterosaurs and dinosaurs4,5,6. Unidirectional flow in birds results from aerodynamic valves, rather than from sphincters or other physical mechanisms7,8, and similar aerodynamic valves seem to be present in crocodilians4,5,6. The anatomical and developmental similarities in the primary and secondary bronchi of birds and crocodilians suggest that these structures and airflow patterns may be homologous4,5,6,9. The origin of this pattern is at least as old as the split between crocodilians and birds, which occurred in the Triassic period10. Alternatively, this pattern of flow may be even older; this hypothesis can be tested by investigating patterns of airflow in members of the outgroup to birds and crocodilians, the Lepidosauromorpha (tuatara, lizards and snakes). Here we demonstrate region-specific unidirectional airflow in the lungs of the savannah monitor lizard (Varanus exanthematicus). The presence of unidirectional flow in the lungs of V. exanthematicus thus gives rise to two possible evolutionary scenarios: either unidirectional airflow evolved independently in archosaurs and monitor lizards, or these flow patterns are homologous in archosaurs and V. exanthematicus, having evolved only once in ancestral diapsids (the clade encompassing snakes, lizards, crocodilians and birds). If unidirectional airflow is plesiomorphic for Diapsida, this respiratory character can be reconstructed for extinct diapsids, and evolved in a small ectothermic tetrapod during the Palaeozoic era at least a hundred million years before the origin of birds.
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We thank J. Dix (Reptile Rescue Service) for the donation of deceased varanid specimens, J. Bourke for assistance with Avizo, and D. Shafer for German translations. This work was supported by an American Association of Anatomists Postdoctoral Fellowship and an American Philosophical Society Franklin Research Grant to E.R.S., National Science Foundation grants to C.G.F. (IOS-1055080 and IOS-0818973) and a generous donation to the Farmer laboratory by S. Meyer.
A volume rendered three dimensional skeleton and segmented surface of the lungs and bronchial tree (left craniolateral view) of a female Varanus exanthematicus generated from a CT scan. The bronchus in which in vivo unidirectional flow was measured is indicated. Abbreviations: cb, cervical bronchus; L1-L10, lateral bronchi 1-10; M1-M11, medial bronchi 1-11.
Microsphere infused saline flowing from lateral bronchus 10 to lateral bronchus 9 in an excised right lung during manual ventilation (60 cc syringe). The microspheres can be seen moving from right to left (caudal to cranial) during inspiration and expiration. Abbreviations: L9, lateral bronchus 9; L10, lateral bronchus 10.